The field of the invention relates to welding methods and apparatus, particularly for manufacturing vinyl window and door frames and the like.
Vinyl window and door products are assembled using either mechanical or welding techniques. The former generally involves the use of screws and/or other fasteners or fastening materials such as adhesives. Vinyl welding is a process in which the material edges of two vinyl frame members are heated at high temperature to a soft state and then pressed together and allowed to solidify into a rigid corner.
Five steps are generally performed in the vinyl welding process: sizing and clamping, burn-off, heating, fusion and cooling. The first step is accomplished by positioning the mitered edges of two frame members against a sizing plate and clamping the frame members in place. The sizing plate is removed and a heating plate inserted in its placed. A plate temperature of about 460.degree. F. is maintained as the two profile sections are moved towards the plate. Such movement continues until about two thirds of the excess material at each end of the respective frame members has been displaced.
The heating cycle, also referred to as the plastification cycle, involves holding the frame profiles in a stationary position against the heater plate for a selected length of time. During this period, the ends of the frames are heated and become soft. At the conclusion of the heating cycle, the heater plate is retracted and the profiles are again moved towards one another. This results in the further compression and fusion of each frame member, thereby displacing the remaining third of the material which was overcut. This movement may be controlled by an internal limit switch.
The last phase of the cycle involves holding the fused material in a stationary position for a length of time. The material cools and solidifies into a rigid corner which can then be removed from the machine.
It is important to allow sufficient time for the heating and cooling cycles as they are critical to establishing a strong bond between frame members. This time will vary depending upon the material employed and the cross sectional areas of the frame members, assuming the temperatures of the heating plate and cooling medium (usually air) are constant. Since there are no reliable inspection methods for determining whether plastification or solidification have been satisfactorily completed, it is generally left to the operator to make these determinations. The skill and experience levels of operators of vinyl welding equipment are variable, which can therefore lead to inconsistent results in the vinyl welding process. Insufficient heating and/or cooling time causes the bond between frame members to be weak. Removal of the frame members from the machine prior to completion of the cooling cycle may result in the distortion of the resulting frame. On the other hand, efficiency may be greatly reduced if too much time is allowed for cooling the fused frame members.
As discussed above, the heating plate is a critical element in the burn-off and heating cycles. Despite the use of materials such as TEFLON (polytetrafluoroethylene) to protect the plate, plastic material tends to accumulate thereon. Replacement and/or cleaning of the plate is accordingly required from time to time, thereby reducing the efficiency of the welding apparatus.